Grad student discovers way of plotting the appearances of El Nino through history

STANFORD -- A Stanford University graduate student in the School of Earth
Sciences has found a way to pinpoint El Nino years in past centuries by
studying the muck at the bottom of the ocean, a technique that should reveal
more about how human activity has affected global climate.

The student, geochemist Julie Kennedy, plans to use the technique to study
sediment layers deposited centuries ago, before anyone kept records of El
Ninos. By identifying El Nino years before and after industrial activity
began to change the earth's atmosphere, she hopes to learn if human activity
may have changed their frequency.

El Nino is an occasional disruption of currents in the Pacific Ocean. It
gets its name - a Spanish term referring to the Christ child - because the
disruption often begins near Christmastime. In El Nino years, the sea surface
in the eastern Pacific warms by several degrees. This contributes to profound
changes in the world's weather, including increased rainfall in the American
Southwest, mild winters in the northern Midwest, drought in Africa, and the
failure of Asian monsoons.

Kennedy studied the molecular remains of single-celled algae in ocean
floor deposits that settle in annual layers like the growth rings of trees.
She was able to trace sea-surface temperature changes year by year through
the 20th century, correctly spotting each recorded El Nino year.

"Of the major El Ninos that have occurred in the 20th century, I've been
able to pick every one up very cleanly," she said.

Kennedy, together with her graduate adviser Simon Brassell - now at
Indiana University - published her results in the May 7 issue of Nature.

Now, she plans to use her technique to examine layers from years in which
no El Nino records exist.

Kennedy studied molecules produced by Emiliania huxleyi, a golden-brown
alga that drifts in the plankton near the ocean's surface. Though each
single-celled individual is less than a thousandth of an inch long, E.
huxleyi is so abundant that collectively it outweighs almost any other
species on earth, Kennedy said.

"Not only is it extremely important, but it's one of the most
accommodating organisms on the face of the earth," making its home everywhere
from near-freezing Arctic waters to warm tropical oceans, she said.

During the 1980s, Brassell - then based in Bristol, England - suggested
that E. huxleyi adjusts to cold water by changing certain chemical bonds in a
class of its molecules called alkenones. The changes help cold-water
alkenones stay fluid at temperatures where warm-water alkenones would stiffen
up, in much the same way that vegetable oil stays liquid at room temperature
while butter solidifies.

Brassell found he could tell at what water temperature the algae lived by
measuring the relative proportion of cold-water and warm- water alkenones
produced by the algae. Work led by Fred Prahl of Oregon State University let
Brassell identify what temperature corresponded to each mix of alkenones.

In her research, Kennedy applied Brassell's technique to alkenones found
in annual layers of dead algae that settle on the ocean floor.

At Kennedy's study site in the Santa Barbara Basin off the California
coast, distinct annual bands accumulate undisturbed because a layer of
stagnant, oxygen-depleted water lies along the ocean bottom. Since
bottom-dwelling animals can't survive without oxygen, their tunnels and
burrows don't mix up the sediment layers.

Kennedy collected sediment cores from the basin, painstakingly scraping
off each year's layer - about a tenth of an inch thick - to analyze the
structure of its alkenones.

She found that in every year in this century when oceanographers recorded
a strong El Nino warming, the sediment record reflected an increase in sea
surface temperature. In other words, Kennedy's work so far has shown that she
can use alkenones to spot El Nino events that oceanographers already knew
about.

"Basically, it was like a calibration. Does this technique work on a
year-by-year basis, and can you use it to identify El Ninos?" she said.

Armed with this technique, Kennedy will begin sampling deeper, older
sediments from the Santa Barbara Basin and the Guaymas Basin in the Gulf of
California this summer.

"Laminated sediments go back tens of thousands of years in the Santa
Barbara Basin," she said, so this El Nino record may extend back to the last
ice age.

Similar layers occur in some sedimentary rocks millions of years old,
raising hopes that scientists may eventually trace ocean temperature changes
even further into the past, Kennedy said.

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